Method of forming conductors within an insulating substrate

ABSTRACT

A method for forming an electrically conductive line between two layers of insulating material and method for connecting the line through both layers of the insulating material to the opposite surfaces is provided. In the method, first, second and third layers of insulating material are provided wherein said first and third layers are separated by said second layer of insulating material which is different in etch rate from the first and third layers. The edge portion of all three layers is exposed and the insulating layer of the second material is selectively etched to remove the revealed edge portion and provide a slot between the first and third layers of insulating material. Also openings are provided in both the first and third layers of insulating material which communicate with the slot and extend respectively through the layers of the first and third insulating material. Thereafter, a conductive material such as tungsten is deposited in the slot and the openings and also on the face of the stacked insulating material. Finally, the excess tungsten is removed from the faces of the insulating material of the first and third layers leaving a conductive line sandwiched between the first and third insulating layers of the material; also metal remains in the openings formed to thereby form conductive studs extending from the line to the opposite surfaces of the insulating material sandwich so formed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the formation of metal linesembedded in substrates and their interconnections through thesubstrates, and more particularly to a method for forming metalconductors imbedded within the substrate and which conductors have metalconnections extending to the opposite surfaces of the substrate. In evenmore particular aspects, this invention relates to a method of forming ametal line between two layers of insulating material which form asubstrate or a portion of the substrate and concomitantly forming metalstuds which extend through the layers of the substrate material tothereby provide electrical interconnections from either side of thesubstrate to the metal connection line embedded within the substrate.

2. Prior Art

As the dimensions of the ULSI circuitry becomes smaller and smaller withthe advancement of technology, it becomes increasingly necessary to formsmaller metal conductors within an insulating substrate and provideprecise connections to surfaces on both side of the insulating substrateall within a very small and closely controlled dimensions. Also,technology is advancing such that electrical devices can be formed onside walls of trenches, which devices must be wired, or connected. Inthe past, it has been conventional practice to form the substrate inseveral different layers depositing the metal layers between theinsulating materials and then etching the exposed metal to the desiredpattern. The etched pattern is connected to various levels by means ofmetal fill vias formed through the insulating layers. While this doeswork quite well in many instances, it is difficult to obtain very closetolerances with small lines requiring precision interconnection to theopposite surfaces. Further, this technique cannot be readily implementedin side wall electrical device technology.

SUMMARY OF THE INVENTION

According to the present invention a method of providing electricalconductive material between two layers of insulating materials isprovided. The method comprises the steps of providing first, second andthird layers of insulating material wherein the second layer ofinsulating material is interposed between said first and third layers.The second layer has an etch rate which differs from that of said firstand third layers and which can be selectively removed without removingthe material of the first and third layers. At least one edge portion ofthe stacked layers of material is exposed. Thereafter an edge portion ofthe second material is selectively removed from the exposed portion toprovide a slot or undercut between the first and third layers ofinsulating material. Thereafter a conductive material is deposited insaid slot and around the exposed edge portions of the first and thirdlayers of insulating materials. Finally the conductive material layingoutside of the slot around the edge portion of the remaining insulatingmaterial is removed to thereby provide a conductive line between twolayers of insulating material.

DESCRIPTION OF THE DRAWING

FIGS. 1a through 1j depict, somewhat diagrammatically, sequentially thesteps of the preferred embodiment of the present invention; and

FIGS. 2a through 2e depict a somewhat modified version of certain of thesteps of the invention from those of FIGS. 1a through 1h.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention can best be understood by referring to the drawings whichdepict sequentially the steps involved in the present invention.

As shown in FIG. 1a, a layer of insulating material 10 preferablysilicon dioxide, is mounted or deposited on a base layer 12. The depositis preferably by chemical vapor deposition (CVD). One preferredtechnique is performed in a parallel plate reactor using 2% silane(SiH₄) with a carrier of He, with an N₂ O flow of 0.75 liters/min. at apressure of 1.9 Torr at 300° Centigrade, the upper electrode beingbiased. The base layer preferably is a silicon substrate which has anetch stop 13 such as Al₂ O₃ coated thereon, by conventional methods suchas reactive sputtering.

A layer of photoresist material 14 is applied on to the insulating layer10, any conventional photoresist material may be used; and, byconventional photo-lithographic techniques, the photoresist material 14is exposed and developed to reveal the underlying substrate 10 in thepattern as shown in FIG. 1b. The structure is then etched to remove theexposed silicon dioxide in the layer 10, the remaining photoresistmaterial acting as an etch mask. The etching is preferably done byreactive ion etching techniques in CHF₃ +CO₂ or CF₄ at a 100 Millitorror less. The etching continues until all the exposed material has beenremoved down to the stop 13. Following this the remaining photoresistmaterial is removed with the resulting structure being as shown in FIG.1c in which a trough or trench 16 has been etched into the silicon withgrooves 18 extending orthogonally therefrom.

At this point a second layer of a second insulating material 20 isblanket deposited over the first insulating layer 10 and the exposedportion of the underlying stop 13 and preferably is planarized. It isessential that the second insulating material be of a differentcomposition or structure from the first insulating material or in someother way differs such that it will react with an etch media which isessentially unreactive or much less reactive to the first insulatingmaterial. Expressed another way, the material 20 must have an etch ratein some etch medium which differs from that of the material 10. In thepreferred embodiment this second insulating material 20 is siliconnitride Si₃ N₄. A preferred method is utilizing plasma enhanced chemicalvapor deposition (PECVD). Preferably it is done in Silane (SiH₄) andammonia (NH₃) with 175 SCCM of Silane and 325 SCCM of NH₃, a presence of2 Torr and at 375° C., with a power of 175 watts. A photoresist material22 is then applied over the insulating material 20 and exposed anddeveloped to reveal the underlying insulating material in the layer 20as shown in FIG. 1d. It will be noted in this embodiment that the edgeof the remaining photoresist material, which will act as an etch mask,on the left side 24 is coincident with the edge of the insulatingmaterial 10 whereas the edge of the remaining photoresist material onthe right side is somewhat offset with the edge of the insulatingmaterial 10 on its right side 26 where the trough 16 is forming a step26 as shown in FIG. 1e.

At this point, the silicon nitride is etched anisotopically preferablyby reactive ion etching in CHF₃ +CO₂ at a low pressure, e.g. 30Millitorr or less. This will selectively anisotopically etch the siliconnitride but will be essentially unreactive with the silicon dioxide inthe insulating layer 10. The resulting structure is as shown in FIG. 1e.

At this point the remaining photoresist material 22 is removed and athird insulating material 28 which must have a different etch rate fromthat of the insulating material in the second layer and preferably isthe same as the insulating material of the first layer, i.e. SiO₂ isdeposited uniformly over the insulating layers 10 and 20 completelyfilling the trough 16. It should be noted at this point that the grooves18 in insulating layer 10 are filled with the silicon nitride materialof the insulating layer 20.

A layer of photoresist 30 is applied on top of the third insulatinglayer 28 and exposed and developed to reveal the underlying silicondioxide in the insulating layer 28 as shown in FIG. 1f. It should benoted that the right side of the remaining photoresist materialcoincides with the right side of the insulating layer 10 and overhangsthe right side of the insulating layer 20 and the left hand side of theexposed photoresist is coincident with the left hand sides of both theinsulating layer 10 and the second insulating layer 20. This is shown inFIG. 1f.

The revealed silicon dioxide of the insulating layer 28 is then etchedpreferably by reactive ion etching techniques as previously described inCHF₃ +O₂ at 200 Millitorr or less which will result in the structureshown in FIG. 1g. In this structure grooves 32 are formed in layer 28which extend orthogonally with respect to the central trough 16. Theremaining photoresist material is then removed.

Following the removal of the photoresist material 30, the siliconnitride insulating layer 20 is isotopically etched by any conventionaltechnique which will etch the silicon nitride but not the SiO₂. Apreferred technique is to utilize hot phosphoric acid (H₃ PO₄). Thiswill undercut the silicon nitride layer forming a longitudinallyextending slot 33 as shown in FIG. 1h. This will also remove the siliconnitride in slots 18, also as shown in FIG. 1h. It should be noted thatthis slot 33 is formed only on the left hand side of the structure wherethe silicon nitride is exposed and not on the right side where the layer28 of the silicon dioxide covers the edge of the silicon nitride in theinsulating layer 20.

At this point a layer of metal 34 is conformably deposited as shown inFIG. 1i. Preferably this metal is tungsten which is blanket deposited byconventional CVD techniques. A preferred method of blanket depositingthe tungsten is to first deposit a seed layer about 500 angstroms thickof tungsten silicide (WSi₂) in a chemical vapor deposition processutilizing WF₆ H₂ and SiH₄ gasses at a pressure of about 150 Millitorrand a temperature of about 450° Centigrade. Following the deposition ofthe seed layer a tungsten metal is deposited also utilizing CVDtechniques in WF₆ H₂ and SiH₄ gasses at about 150 Millitorr and 450°Centigrade. These depositions can take place in a coldwall reactor, suchtechnique for deposition of tungsten being well known in the art.

Finally, the tungsten is subjected to a wet chemical etch of diluteperoxides which stops on the seed layer of WSi₂. The seed layer isetched in a 20/1 ratio of nitric acid and ammonium fluoride. This etchwill not etch tungsten or oxides. These combined etches will remove thetungsten and WSi₂ on the exposed edges of the stack of insulating layers10 and 28, and the etching is stopped when only the tungsten and WSi₂which is deposited in the slot 33 and in the groves 18 and 32 stillremains. This final structure is shown in FIG. 1j, the resultingstructure being an electrical line 36 disposed between insulating layers10 and 28 with upwardly extending studs 38 to form electricalconnections extending through openings in the insulating layer 28 anddownwardly extending studs 40 to form electrical connections throughopenings in the insulating layer 10.

At this point, the basic wiring plan is established and the structure soformed can be utilized for various wiring schemes on various devices.For example, this technique is particularly useful if one wishes to havesidewall formed semiconductor devices, since it provides a very viableand well designed technique for connecting to sidewall formed devices.

Further it should be understood that there are many modifications of thevery specific technique which has been described. For example, if onewished to have wiring on both sides of the opening then the right sideof the opening could be formed similarly to the left side which wouldprovide wiring on that side as well.

Further, if it is anticipated or determined that there are or may beproblems controlling the etching of the silicon nitride layerisotopically by the hot phosphoric acid to form a uniform undercut asshown in FIG. 1h, then the process can be slightly modified as follows:

In the step of applying the photoresist 22 over the silicon nitride 20as shown in FIG. 1d, the photoresist is applied in a band or strip aboutone micron wide or whatever size is desired for the final electricalline width as shown in FIG. 2a. Hence, when the anisotropic etch in CHF₃+CO₂ is performed, the exposed silicon nitride to the left of thephotoresist 22 is also removed as well as that in the trough 16 leavinga silicon nitride band 20 of about 1 micron in width below thephotoresist 22 as shown in FIG. 2b. In the subsequent steps thephotoresist 22 is removed, and the silicon dioxide 28 is deposited aspreviously described, and the photoresist 30 is applied thereon andpatterned as previously described, as shown in FIG. 2c, and the SiO₂ 28etched to form the structure shown in FIG. 2d. As shown in FIGS. 2cthrough 2e, the layer of silicon dioxide 28 also fills in to the left ofthe silicon nitride layer 20; thus, when the hot phosphoric acidisotopically etches the silicon nitride 20 to produce the structureshown in FIG. 2e, the etching will automatically be stopped when all ofthe silicon nitride has been etched to reveal the layer of silicondioxide 28 adjacent the silicon nitride. The metal can then be depositedand etched in the same manner as described in the previous embodimentand shown in FIG. 1i and 1j.

While the invention has been described, with a certain degree ofparticularity, various adaptations and modifications can be made withoutdeparting from the scope of the invention as defined in the appendedclaims.

What is claimed is:
 1. A method of providing an electrically conductivematerial between two layers of insulating material, said methodcomprising the steps of;providing first, second and third layers ofinsulating material, wherein said first and third layers are separatedby said second layer, said second layer having an etch rate that differsfrom that of said first and third layers, exposing at least one edgeportion of said first, second and third layers of insulating material,selectively removing the exposed edge portion of said second insulatingmaterial to provide a slot between said first and third layers ofinsulating material, thereafter depositing conductive material in saidslot and around said exposed edge portions of the first and third layersof insulating materials, and removing the conductive material layingoutside said slot on said exposed edge portions.
 2. The invention asdefined in claim 1 further characterized by forming at least one openingthrough at least one of said first and third layers of said insulatingmaterial, each of said openings intersecting said slot, and depositingconductive material in each opening so formed.
 3. The invention asdefined in claim 2 wherein said openings are formed through both firstand third layers of insulating material.
 4. The method as defined inclaim 1 wherein said first and third insulating layers are the samematerial.
 5. The invention as defined in claim 4 wherein said first andthird layers are silicon dioxide.
 6. The invention as defined in claim 5wherein said second insulating material is silicon nitride.
 7. Theinvention as defined in claim 1 wherein the conductive material istungsten.
 8. A method of forming an electrically conductive materialbetween two layers of insulating material comprising the stepsof;providing a first layer of insulating material, forming a pattern insaid first insulating material which includes a trough extendinglongitudinally therein, providing a second insulating material overlyingthe first insulating material and disposed in said trough, said secondinsulating material having an etch rate which differs from that of saidfirst insulating material, selectively removing the second insulatingmaterial that is disposed within said trough, providing a third layer ofinsulating material overlying said second layer of insulating materialand disposed in the trough formed in said first and second layers ofinsulating material, said third insulating material having an etch ratedifferent from that of said second insulating material, selectivelyremoving said third insulating material from said trough to therebyexpose the edges of said three layers of insulating material on at leastone side of said trough, selectively removing the edge portion of saidsecond insulating material to form a slot between said first and thirdlayers of insulating material, thereafter depositing a conductivematerial in said slot and on said exposed edges of said insulatingmaterials, and selectively removing said conductive material on saidexposed edges, whereby to form a conductive line between two layers ofinsulating material.
 9. The invention as defined in claim 8 furthercharacterized by forming openings in at least one of said first andthird layers of said insulating material which openings intersect withsaid trough, and depositing said conductive material in said openingswhereby to form electrical connections through said selected insulatingmaterial to the conductive line.
 10. The invention as defined in claim 9wherein openings are formed through both said first and third insulatinglayers and conductive material is deposited in said openings formed,whereby to form electrical interconnections through both said first andsecond insulating layers to said conductive line.
 11. The invention asdefined in claim 8 wherein said first and third insulating materials arethe same material.
 12. The invention as defined in claim 11 wherein saidfirst and third insulating materials are silicon dioxide.
 13. Theinvention as defined in claim 12 wherein said second insulating materialis silicon nitride.
 14. The invention as defined in claim 8 wherein thethird layer of insulating material also has a portion thereof depositedonto said first layer of insulating material and lying adjacent saidsecond layer of insulating material;whereby said portion of said thirdinsulating material on said first insulating material acts as a stopwhen removing said second insulating material from between said firstand third layers of insulating materials.